This invention relates generally to analog-to-digital and digital-to-analog converters and, more particularly, to techniques for increasing the accuracy of digital-to-analog converters.
Analog sources of information, such as voice communications and sound recordings, are increasingly being transmitted and recorded in digital form in order to improve the signal to noise ratio of the received or reproduced signal. However, to fully realize the benefits of digitization, the analog information must be accurately converted into digital form and then back again into analog form. Unfortunately, analog-to-digital (A/D) and digital-to-analog (D/A) converters introduce several types of errors into a signal, including resolution or quantization errors and linearity errors.
Quantization errors are an inherent result of the digitization process, since a continuous analog signal is being sampled and quantized into a finite number of discrete levels. For example, a flash A/D converter generally includes a bank of resistors which divide a reference voltage into a number of discrete voltage levels. The analog input voltage is compared to these discrete voltage levels to convert the analog signal into digital form. Similarly, a D/A converter generally includes a bank of constant current sources which are switched into and out of an output bus in accordance with the digital input signal, thus converting the digital signal into an analog current.
Quantization errors can be reduced by increasing the resolution of the converters, either by increasing the number of resistors and current sources or by oversampling the signal. Oversampling is a technique in which a signal is sampled at a frequency that is much higher than the sampling frequency required by Nyquist's theorem. Nyquist's theorem generally requires that the sampling frequency be at least two times higher than the highest frequency component of the signal.
In contrast, linearity errors are caused by errors in the individual resistors and current sources. For example, in a D/A converter, the same current sources are connected to the output bus for a particular digital input code. If each current source supplies slightly different amounts of current, the analog output current will have an error that depends on the digital input signal. This input-dependent error is a linearity error which can be reduced by using well-matched, high-precision current sources. However, these types of current sources are very costly.
A paper by L. Richard Carley and John Kenney entitled "A 16-Bit 4th Order Noise-Shaping D/A Converter" discloses a technique for reducing linearity errors in D/A converters. In this technique, the current sources are randomly connected to the output bus in accordance with the digital input signal. This technique averages out the errors in the individual current sources, but requires a very large number of switches. Accordingly, there is still a need for a simple technique for improving the accuracy of D/A converters. The present invention clearly fulfills this need.